1. Field of the Invention
The present invention relates to a memory system including a nonvolatile semiconductor memory and a method for controlling a nonvolatile semiconductor memory.
2. Description of the Related Art
As an external storage device used in a computer system, an SSD (Solid State Drive) mounted with a nonvolatile semiconductor memory such as a NAND-type flash memory attracts attention. The flash memory has advantages such as high speed and light weight compared with a magnetic disk device.
The SSD includes a plurality of flash memory chips, a controller that performs read/write control for the respective flash memory chips in response to a request from a host apparatus, a buffer memory for performing data transfer between the respective flash memory chips and the host apparatus, a power supply circuit, and a connection interface to the host apparatus (e.g., Japanese Patent No. 3688835).
Examples of the nonvolatile semiconductor memory include nonvolatile semiconductor memories in which a unit of erasing, writing, and readout is fixed such as a nonvolatile semiconductor memory that, in storing data, once erases the data in block units and then performs writing and a nonvolatile semiconductor memory that performs writing and readout in page units in the same manner as the NAND-type flash memory.
On the other hand, a unit for a host apparatus such as a personal computer to write data in and read out the data from a secondary storage device such as a hard disk is called sector. The sector is set independently from a unit of erasing, writing, and readout of a semiconductor storage device.
For example, whereas a size of a block (a block size) of the nonvolatile semiconductor memory is 512 kB and a size of a page (a page size) thereof is 4 kB, a size of a sector (a sector size) of the host apparatus is set to 512 B.
In this way, the unit of erasing, writing, and readout of the nonvolatile semiconductor memory may be larger than the unit of writing and readout of the host apparatus.
Therefore, when the secondary storage device of the personal computer such as the hard disk is configured by using the nonvolatile semiconductor memory, it is necessary to write data with a small size from the personal computer as the host apparatus by adapting the size to the block size and the page size of the nonvolatile semiconductor memory.
The data recorded by the host apparatus such as the personal computer has both temporal locality and spatial locality (see, for example, David A. Patterson and John L. Hennessy, “Computer Organization and Design: The Hardware/Software Interface”, Morgan Kaufmann Pub, 2004 Aug. 31). Therefore, when data is recorded, if the data is directly recorded in an address designated from the outside, rewriting, i.e., erasing processing temporally concentrates in a specific area and a bias in the number of times of erasing increases. Therefore, in the NAND-type flash memory, processing called wear leveling for equally distributing data update sections is performed.
In the wear leveling processing, for example, a logical address designated by the host apparatus is translated into a physical address of the nonvolatile semiconductor memory in which the data update sections are equally distributed.
When a large-capacity secondary storage device is configured by using the NAND flash memory, if a unit of data management is a small size (e.g., a page size) when the address conversion is performed, the size of a management table increases and does not fit in a main memory of a controller of the secondary storage device, and the address conversion cannot be performed at high speed. In this way, the size of the management table inevitably increases according to an increase in capacity of the NAND flash memory as the secondary storage device. Therefore, there is a demand for a method for reducing a capacity of the management table as much as possible.
As explained above, when a data erasing unit (a block) and a data management unit are different, according to the progress of rewriting of the flash memory, blocks are made porous by invalid (non-latest) data. When the blocks in such a porous state increases, substantially usable blocks decrease and a storage area of the flash memory cannot be effectively used. Therefore, processing called compaction for collecting valid latest data and rewriting the data in different blocks may be performed (see, for example, Japanese Patent Application Laid-Open No. 2005-22550). In this compaction processing, because processing time is long depending on the method, there is a demand for a method for reducing processing time of the compaction.